Lens Speed (Fast Glass) = Depending on the build of the lens, Its maximum aperture setting will determine the amount of light that can enter your camera at any one time. Laymans terms, Usaly the lower numbered Fstop of the lens I.E 100mm F2.8 will be faster than a 100mm F4 and so on 100mm F5.6 (prime lens) Fixed focal length. Or in the case of a Zoom lens, a 18-50mmF2.8 will let more light in across the Stopping range of the camera than a 18-50mm F4-5.6. It's better for a all round lens. So with the seting sun and changing focal lenghts, the camera Fstop may alter for DOF, but the Fstop, apperture or amount of light capible of entering will be the same and also higher with a F2.8 against say F4-5.6.
Aperture on the lens is the blading system that opens or resricits the light entering your cameras sensor. Think the opening of James bond movie, when he turns to shoot, you see that circular blading, like looking through a camera lens, Lower is higher and vise wersa.
This is tricky if you want to take dedicated low light landscape with longer/slower shutter speeds, not always needed on say a 10-22mm lens (UWA Ultra wide angle, for great landscape and building sceens). But for a general walk about general use lens, you are better of getting quality glass (build quality and lower Fstop numbering I.E F2.8) ISO and Shutter duration, and so on, all come into play. Fixed aperture across complete focal lenght is best say 18-50mm @ F2.8 Lens in Aperture Priority mode on the camera, the body will do the rest.
Firstly work out your main photographic needs around what body do what you want.
?. Main intended use (is it for speed or portriature shooting)
?. FPS Frames per second
?. Video requirements if needed
?. Full Frame or Not (this will alter your lens thinking quite a lot)
?. weather sealing (will haevily way on price)
Then work on lenses, usally you could get away with 2, say something around but not restricted to 15/17/18MInumum - 50/55/70/75/80Maximum Focal Zoom odd (several Canon and Non Canon lens here, and say the 70-200mm L F2.8 or lighter but just as sharp if not sharper 70-200mm L F4.
Would be better of there rather then looking at a prime lens. Fixed focal lens (I.E 150mm F2.8) In other words, you need to move closed or further away from subject to chage focal range. But very good optics in most case's in prime lens.
Have Fun...